Hydrocarbon separation process employing crystalline aluminosilicates



Aug. 12, 1969 D E. COOPER 3,461,065

HYDROCARBON SEI ARATION PROCESS EMPLOYING CRYSTALLINE ALUMINOSILICATESFiled July 5, 1967 United States Patent 3,461,065 HYDROCARBON SEPARATIONPROCESS EMPLOY- ING CRYSTALLINE ALUMINOSILICATES David E. Cooper,Greenville, S.C., assignor to Texaco Inc., New York, N.Y., a corporationof Delaware Filed July 3, 1967, Ser. No. 650,857 Int. Cl. C10g 25/04 US.Cl. 208-310 5 Claims ABSTRACT OF THE DISCLOSURE Background of theinvention This invention relates to improvements in an absorbent methodof separating straight chain hydrocarbons from nonstraight chainhydrocarbons with crystalline zeolitic aluminosilicates, commonly calledmolecular sieves. More particularly, it relates to improvements in ahydrocarbon separation process employing molecular sieves to producehigh purity straight chain hydrocarbons.

Zeolitic molecular sieves are a group of natural and synethticaluminosilicates whose unique crystalline structure upon dehydrationproduces a network of crystallographic unit cells interconnected bypores having a precise uniform diameter. These pores permit a sieving orscreening action in the molecular size range thereby permitting theseparation of molecules based on their average particle diameters. Theterms molecular sieves, crystalline zeolites and aluminosilicates referto the same general class of adsorbent materials and are usedinterchangeably herein.

A particularly striking examble of using molecular sieves to separatemixtures based on the average particle diameter of the components is theseparation of straight chain hydrocarbons from a mixture of straightchain and nonstraight chain hydrocarbons by contacting the mixture withmolecular sieves having uniform pore openings of about 5A. Theseparticular molecular sieves, referred to as a Type 5A sieve, permit thepassage of the straight chain hydrocarbons through the pores into thesieve cages where they are adsorbed while rejecting the nonstraighthydrocarbons.

Prior art processes employ a cyclic operation when separating thestraight chain hydrocarbons from the nonstraight chain hydrocarbons in ahydrocarbon stream with molecular sieves. The straight chainhydrocarbons are adsorbed within the sieve cages of the molecular sievesand recovered from the sieves in a desorbing step which usually isaccomplished by a change in temperature, :pressure, the use of adesorbing fluid or a combination thereof. The straight chainhydrocarbons are seldom recovered as a pure product-a small quantity ofnonstraight chain hydrocarbons is usually present as an impurity. At theend of the adsorption step, some of these contaminants are retained inthe void spaces of the molecular sieve bed while the more polar of thesenonstraight chain hydro carbons are adsorbed on the surface of theindividual sieve particles. Consequently they are removed together withthe adsorbate in the sieve cages during the desorption step. To improvethe purity of the straight chain hydrocarbon stream the bed is sometimespurged following the "ice adsorption step to remove the hold-up ofnonstraight chain hydrocarbons from the sieve bed. Often the purgingstep is merely the first portion of the desorbing step with the initialeflluent, containing a high concentration of the nonstraight chainhydrocarbons, being segregated from the rest of the efiluent whichcontains substantially all of the adsorbed straight hydrocarbons. Sincesome straight chain hydrocarbons are unavoidably removed during purging,the length of the purging step must be balanced between a short purgeremoving from the bed a major portion of the nonstraight chainhydrocarbons but substantially none of the adsorbed straight chainhydrocarbons and a long purge removing substantially all the nonstraightchain hydrocarbons from the bed but also a significant quantity oftheadsorbed straight chain hydrocarbons. p v

It is known then that three variables in the molecular sieve separationprocess affect the purity of the straight chain hydrocarbon product: theamount of straight chain hydrocarbons adsorbed by the molecular sieves,the amount of nonstraight chain hydrocarbons adsorbed on the surface ofthe sieves and retained in the void spaces of the sieve bed and theseverity of the purging step employed between the adsorption anddesorption steps.

It is also known that in a molecular sieve-hydrocarbon separation awide-cut feed may be employed Where the number of carbon atoms permolecule may have a spread of 10 or more carbon atoms even thoughseveral narrowcut straight chain hydrocarbon product streams arerequired. A simple fractionation of the wide-cut straight chainhydrocarbon product will yield the several narrow cut streams desired.However, although the purity of the wide-cut stream may be -995 weightpercent or higher, the several cuts obtained therefrom will not be ofequal purity. Some will have a lower purity while others will be purerthan the wide-cut product. The nonstraight chain hydrocarboncontaminants, which include the surface adsorbed polar compounds, suchas dinuclear aromatics, have a narrower boiling range than the feedstockand therefore are not uniformly distributed in the narrow-cut productstreams but are found usually in only one of the straight chainhydrocarbon product streams. This particular narrow-cut product stream,referred to hereinafter for simplicity as product fraction X has a lowerstraight chain hydrocarbon purity than the other product fraction orfractions. When producing several narrow-cut straight chain hydrocarbonsit is often desirable to offer all the commercial products withsubstantially the same degree of purity.

In order to increase the purity of the product fraction X stream amodification of process conditions and variables will achieve a limitedmeasure of success. Although increased purging severity will improve thepurity of the wide-cut straight chain hydrocarbon stream with aconcomitant improvement in product fraction X, very severe purgingconditions are required to produce a product fraction X with a purityabove about 98 weight percent. These severe purging conditions aresomewhat undesirable because they cause a significant loss of straightchain hydrocarbons in the purge effluent and produce a higher puritythan is normally required in narrow cut fractions other than productfraction X. In addition, since the adsorptive capacity of the molecularsieves gradually declines with operating time between regenerations, apoint is reached in a relatively short time beyond which a very highpurity product fraction X cannot be produced regardless of purgingseverity.

Rather than processing a wide-cut feed with subsequent fractionation ofthe wide-cut straight chain hydrocarbons into several narrow cuts, it ispossible to use a so-called blocked-out operation where a number ofnarrow cut feedstocks would be processed in the same equipment but inseparate and distinct operations. Although this offers the advantage ofbeing able to regulate processing conditions and purging severity toproduce the desired purity for each product stream, the attendantproblems of segregating feedstocks and upset" conditions while switchingfrom one feedstock to another resulting in the reprocessing of off-specproducts do not make this technique the most eflicient processingscheme.

It would be desirable therefore to process a wide-cut hydrocarbonfeedstock containing both straight chain hydrocarbons and nonstraightchain hydrocarbons producing a multiplicity of narrow-cut straight chainhydrocarbon product streams wherein the purity of the most impure streammay be substantially improved without a significant change in the purityof the other product stream or streams.

Summary of the invention I have found an improved process for separatingwidecut mixtures of straight chain hydrocarbons and nonstraight chainhydrocarbons into several narrow cut straight chain hydrocarbon streamswherein the purity of the lower purity straight chain hydrocarbonproduct stream is substantially improved by recycling a portion of thisstream and combining it with the wide-cut fresh feed.

When a bed of molecular sieves is being utilized to separate a wide-cutfresh feed of hydrocarbons into a wide-cut stream of high puritystraight chain hydrocarbons, a fractionation of this product into two ormore narrow cut fractions produces streams of unequal purity because theimpurities in the wide-cut product have a narrow boiling range and tendto concentrate in one of the narrow-cut product streams, designatedproduct fraction X. By recycling a portion of product fraction X andcombining it with the fresh feed for reprocessing, the degree of purityof the portion of product fraction X recovered as product will besignificantly increased with no substantial change in the purity of theother product stream or streams.

Since the recycled product fraction X is itself a high purity stream ofstraight chain hydrocarbons, both the amount of nonstraight chainhydrocarbons in the combined streams being charged to the bed ofmolecular sieves and the amount of nonstraight chain hydrocarbonssurface-adsorbed on the sieves remain essentially unchanged. Therefore,the ratio of straight chain hydrocarbons adsorbed within the sieve cagesto nonstraight chain hydrocarbons surface adsorbed within the boilingrange of product fraction X is increased resulting in a higher purityproduct fraction X at given purging conditions. The degree of purityimprovement is determined by the amount of product fraction X recycled.By varying the quantity of recycle, product fraction X may be producedwith a purity lower than, equal to or higher than that of the othernarrow cut straight chain hydrocarbons.

Brief description of the drawing The present invention will be morereadily understood by reference to the accompanying drawing which is aschematic flow diagram of the process of the invention wherein a part ofthe lower purity narrow cut product of straight chain hydrocarbons isrecycled for reprocessing with the fresh feed.

Description of the preferred embodiment My invention may be understoodfrom the following detailed description, taken with reference to theaccompanying drawing, which illustrates diagrammatically the preferredembodiment for practicing the process of my invention.

The drawing illustrates a process of separating'a widecut mixture ofstraight chain hydrocarbons and nonstraight chain hydrocarbons with abed of molecular sieves into several narrow cut streams of straightchain hydrocarbons wherein the purity of the more impure stream issubstantially improved.

The operation is a cyclic one which alternates between an adsorptionstep and a desorption step. Optionally, a purging operation may followeach adsorption and desorption step. As is well known in the art, thecyclic operation may be made continous by using a series of vesselscontaining beds of molecular sieves together with the necessaryinterconnecting piping and valves. Although only one sieve bed vessel isshown in the drawing, the following description is based on the stepsconcerning this particular vessel, it being understood however, thatwith a multiplicity of beds a feedstrearn is continuously beingintroduced into the system, product streams are continuously beingwithdrawn and recycle streams are continuously flowing from vessel tovessel.

In the first phase of the cyclic operation a mixture of straight chainhydrocarbons and nonstraight chain hydrocarbons having a wide boilingrange of 330 to 510 F., for example, passes through line 10 into vessel12 containing a fixed bed of molecular sieves having a pore diameter ofabout 5 A., so-called Type 5A molecular sieves. The sieves are incondition to adsorb straight chain hydrocarbons having been desorbed inthe step preceding the adsorption step being described herein. When thedesorption was accomplished by a change of pressure or temperature, thesieve cages and the void spaces in the bed will be substantially free ofhydrocarbon material. If a nonadsorbable hydrocarbon stream was used asa desorbent medium, the void spaces will be substantially filled withdesorbent whereas if the desorbent was an adsorbable material the sievecages and the void spaces in the bed will also be filled with thedesorbent.

The various desorption techniques are well known in the art and theparticular method employed in a given molecular sieve separation processdoes not restrict the usefulness of the present invention. In thefollowing description, the use of a desorbent fluid has been assumed.

As the feed passes through the sieve bed, straight chain hydrocarbonsare adsorbed in the sieve cages of the molecular sieves. The eflluentflowing from vessel 12 through line 14 is substantially depleted ofstraight chain hydrocarbons. Any desorbent that may have been present inthe sieve bed will be carried out from the sieve bed by the efliuent forrecovery and reuse, leaving the efiluent as an essentially nonstraightchain hydrocarbon product. When the adsorptive capacity of the sieves isfully utilized the feed stream is switched from sieve bed vessel 12 toanother of the sieve beds and the desorption step commences in vessel12.

The adsorbed hydrocarbons may be desorbed by a displacement techniqueusing an adsorbable material which is less strongly adsorbed than thematerial being desorbed, i.e., a straight chain hydrocarbon having alower molecular weight than the straight chain hydrocarbons adsorbed onthe sieve. For example, when a C -C hydrocarbon feedstock is processed,n-heptane may effectively be used as a desorbent. The desorption streamflows into sieve vessel 12 from line 16. The resulting desorptioneffluent is withdrawn from vessel 12 by means of line 18. The eflluentcontains the desorption medium together with the desorbed straight chainhydrocarbons recovered from the sieve bed, and a small quantity ofnonstraight chain hydrocarbons which was adsorbed on the surface of thesieves and held up in the void spaces of the bed. The eifluent flowsthrough line 18 into fractionator 20 where the desorbent medium isrecovered as an overhead stream flowing through line 16. The wide-cutstraight chain hydrocarbons are recovered as a bottoms product flowingthrough line 22 to a second fractionator 24 where they are separatedinto the desired narrow cut straight chain hydrocarbon streams. Wheretwo product streams are desired, one is recovered as an overhead streampassing through line 26 while the second is a bottoms product recoveredthrough line 28. Both of the narrow-cut fractions Will contain some ofthe 6 nonstraight chain hydrocarbons recovered from the bed. which isrecycled. Although the absolute purity of product Since the surfaceadsorbed hydrocarbons will tend to confraction X is not directlydependent on the purity of the centrate in one of these narrow-cutproduct streams, that other narrow cut product stream the proportion ofprodstream will contain the major portion of the nonstraight uctfraction X which is recycled will influence the relative chainhydrocarbon impurities. This stream, designated purities of these narrowcut product streams. Thus, as product fraction X has a significantlylower purity than 5 shown in Table I, when 97 weight percent pure,wide-cut the other stream. Where it is the bottoms product which productstream is fractionated, the product fraction X contains the bulk of theimpurities, a portion of this stream is 95 weight percent pure while theother narrowstream passes from line 28 into line 30 and is recycled cutproduct stream is 98 weight percent pure. By varying back to andcombined with the fresh feed flowing in line the amount of productfraction X being recycled, product 10. Alternately, where the overheadproduct is the more 10 fraction X of any desired purity, within adesirable range, impure, a portion of this stream will be recycledinstead. for example from 95 up to about 99 weight percent can Theamount of recycle is adjusted to produce product be obtained when thewide-cut product stream purity stream 32 having the desired purity.ranges from 98 to 99 weight percent and the other nar- The following isa description by way of example of a row-cut product stream purityranges from 98 to 99 method of carrying out the process of the presentinweight percent. vention. Those skilled in the art will appreciate thatnarrow cut A hydrocarbon feed stream, containing weight perstraightchain hydrocarbons having other purity ranges cent C -C straight chainhydrocarbons together with 75 can be produced by modifying theprocessing conditions weight percent C C nonstraight chain hydrocarbonsof of the molecular sieve separation and the quantity of prodequalboiling range, is processed in two runs, the first dem- 20 Hot fractionX which is recycled.

TABLE L-PRIOR ART PROCESS Stream No 10 14 22 26 32 Non-Normal Gin-C12Gil-C15 Cit-C15 Paraffin Wide Cut n-Paratfin n-Paratfin n-ParafiinStream Description Fresh Feed Product n-Paraflin Product Product RecycleRate, lbs/hr 1, 000 743 257 204 53 n-Parafiin Content, Wt. percent 25 9798 95 Composition, 1bs./hr.:

C o-C13 Il-PEI'E-ffinS 200 014 015 n-Paraffins a l 50 Total Non-NormalParatfins 7 50 Rate, lbs/hr 1, 000 nParafiin Content, wt. percent 25Composition, lbs./hr.:

C ty-C n-Parafiins 200 CW0]; n-Paratfins 50 Total Non-Normal Paraflins750 745 7 4 1 2 onstrating the prior art and the second the method of myI claim: invention. Reference to the drawing will be used to dem- 40 1.A hydrocarbon separation process which comprises: onstrate both runs. Inillustrating the prior art separation (1) passing a mixture comprising ahydrocarbon feedprocess, the straight chain hydrocarbons are separatedStock containing straight chain hydrocarbon and nonfrom the nonstraightchain hydrocarbons by means of a straight chain hydrocarbons intocontact with a bed bed of molecular sieves contained in sieve vessel 12.Then of crystalline aluminosilicates effecting a separation a stream ofn-heptane is used to remove the adsorbed 5 of said mixture into a firstfraction consisting essenstraight chain hydrocarbons together with thesurfacetially of straight chain hydrocarbons and containing adsorbednonstraight chain hydrocarbons which are a small quantity of nonstraightchain hydrocarbons mostly C -C dinuclear aromatics. The efiluent passesto and a second fraction consisting essentially of nonfractionator 20where the hcptane is recovered as the overstraight chain hydrocarbons;head and the C C hydrocarbons as the bottoms. This (2) separating saidfirst fraction into a third fraction bottoms stream i Sgparated i t C Cd c c conslstmg essentially of straight chain hydrocarbons fractions infractionator 24. The overhead product of and contalmng a major portionof the nonstraight C -C straight chain hydrocarbons is 98 weight percentchain hydrocarbons of said first fraction, said non- Pure Whereas the C143 i h h i h d b b tstraight cham hydrocarbons having substantially thetoms product is about 95 weight percent pure. Same boiling Poiht rangeas the Straight Chain y In the run demonstrating the process of myinvention carbons 1n said third fraction, and a fourth fraction theseparation is performed as in the prior art process exonslstmgessentially of straight chain hydrocarbons t h t a major ti f the c ctraight h i and containing a minor portion of the nonstraighthydrocarbon stream recovered as the bottoms in fractionohfilflhydrocarbons of Said first fraction, Said 11011- ator 24 is recycledthrough line 30 back to be combined stl'alght F hydrocarbons havingSubstantially the with the fresh feed. When -70 percent of the C C Sameholhng Polht range as the Straight ohahl y fraction is recycled, thepurity of the C -C product i carbons in the fourth fraction, said thirdfraction increased to about 98 weight percent so as to equal that of oSold fourth fraction having different boiling the C -C product. Byadjusting the amount of recycle Polnt the purity of the C C product canbe made lower or 65 p h a Portion of Said third raction n o contact hi hh 93 i h percent, with said bed of aluminosilicates as a portion of saidTable I below presents a material balance of these two feedstockrunstogether with the composition of the several streams A Process accordingto claim 1 e e n the feedstock which are identified by the numberassociated with that Comprises C1D"C15 Straight chain hydrocarbons dstream in the fiowplan drawing. The straight chain hydrolo- 15hohstl'aight Chain hydrocarbons, Said third fr carbons are referred toas n-parafiins; the nonstraight tion comprises C14C15 hydrocarbons and af rth fracchain hydrocarbons as nonnormal paraflins. tion comprises ur-13 hydrocarbons- The ultimate purity of the product fraction X stream AP ss according to claim 2 wherein the separais dependent principally onthe efliciency of the molecular tion means in step (2) in distillation.

sieve separation process and the proportion of this stream 4. A processaccording to claim 2 wherein the percentage of straight chainhydrocarbons in the fractions is as follows:

in the first fraction, at least 97 Weight percent; in the thirdfraction, at least 95 weight percent and in the fourth fraction, atleast 98 weight percent. 5. A process according to claim 3 wherein thepercentage of straight chain hydrocarbons in the fractions is:

in the first fraction, between 98 and 99 weight percent;

in the third fraction, between 95 and 99 weight percent and in thefourth fraction, between 98 and 99 weight percent.

8 References Cited UNITED STATES PATENTS 4/1968 Powers et a1. 208-310HERBERT LEVINE, Primary Examiner US. Cl. X.R.

